A radial piston pump having a pump shaft rotatably provided within a housing, a rotor connected with the pump shaft, and a piston reciprocally provided within a cylinder formed in the rotor in such a manner that the piston is reciprocated in accordance with the rotation of the rotor has been known (U.S. Pat. No. 3,756,479). Since the top end of the piston of such conventional type of radial piston pump abuts toward an inner surface of an inner race and since the top end of the piston slides along with the inner surface of the inner race, the conventional type of radial piston pump has such disadvantages that the top portion of the piston and the inner surface of the inner race wear and an eccentric noise may generate within the pump portion.
In order to solve these disadvantages, the radial piston pump employs a shoe between the top end of the piston and the inner surface of the bearing inner race (U.S. Pat. No. 3,872,271). The shoe is used for reducing the abrasion. The shoe, however, caused the disadvantage that the shoe increases the production cost and the shoe requires another assembling step. Furthermore, since the shoe also requires the limiting mechanism for protecting the shoe from dropping off the rotor, the radial piston pump employing the shoe is a complex structure.
A ball is employed at the top end of the piston in order to reduce the abrasion of the top end of the piston and the inner surface of the inner race (U.S. Pat. No. 4,555,223). Since the inner diameter of the ball should be smaller than the inner diameter of the piston, the stress on the abutting area where the ball contacts with the inner race is great. Thus, the ball cannot to use for a long period of time. Furthermore, since the piston is not biased toward the ball, the ball may become spaced from the piston for example, when the temperature of the working fluid is cold and viscosity of the working fluid is high. The pump cannot work effectively when the ball aparts from the piston.